1. Field of the Invention
The present invention relates to an interdigital transducer for a surface wave filter which comprises split-finger pair electrodes and single finger electrodes, of which the split-finger pairs are disposed in accordance with a predetermined local periodic distribution, and, particularly, a low-loss filter having at least two input interdigital transducers, having at least two output interdigital transducers, and having at least one pair of coupling transducers.
2. Description of the Related Art
The transfer characteristic of a surface wave filter is determined by the geometry of the electro-acoustic interdigital transducers contained in the filter and disposed on a piezoelectric substrate, and by the topology of their arrangement. The acoustic wave which is excited in one (input) interdigital transducer can be disturbed by reflections of an electric and/or mechanical nature during its propagation in the surface of the substrate underneath the fingers of these transducers. It is known that this leads to distortions in the transfer characteristic, displacement of the center frequency and reduction in the bandwidth of the filter concerned. A more profound reason for such disturbances is the geometric deviation of the centers of excitation and of the centers of reflection of the individual transducer from one another. Such disturbances become noticeable, in particular, in the case of substrates having a high coupling factor, with relatively long transducers and/or with highly reflective finger edges.
To eliminate such defects, it has been known for a long time to construct respective interdigital transducers as split-finger transducers for avoiding such disturbances. In the case of split-finger interdigital transducers, the reflections occurring there, too, at the finger edges are suppressed by destructive interference. However, it has been found that (even) split finger interdigital transducers or filters having such transducers exhibit asymmetry in the transfer characteristic.
From Japanese patent application 53-20840, an interdigital transducer provided for a surface wave filter is known which, in addition to interdigital split-finger pairs, also has interdigitally arranged single fingers.
The purpose of the transducer with split fingers specified in the reference patent is to reduce those components of triple transit (echo) signals (hereinafter known as T.-T. signals) which occur in a known split-finger transducer when this transducer is well matched to the characteristic impedance of the filter. It is known that the split-finger transducer only has a high degree of suppression of the triple-transit signal with a "poor" match. Insertion of the single fingers into the transducer of the reference patent serves the purpose of reducing the T.-T.-signal, without influencing the transfer function of the transducer or of the filter by this action. This is indicated, in particular, by the circumstance in the case of transducers of the Japanese reference, the single fingers that are located in place of split-finger pairs can be both electrically connected and electrically unconnected (see FIGS. 3 and 4 of the Japanese document). If the connected or also non-connected single fingers were to be omitted in the transducer of the above-cited Japanese Application, this would not alter the characteristic of the transfer function but the transfer function would only exhibit additional ripples due to the T.-T.-signal occurring. If a transducer of the above-cited Japanese application has unconnected single fingers (as in FIG. 4 of the Japanese reference), its attenuation is changed.
The operation of the Japanese disclosed transducer is based on phase shifts provided in the transducer which, in turn, are based on the insertion of the single fingers in place of omitted split fingers.
As has already been mentioned above, transducers with split-finger construction have long been known. However, their application is being handled very restrictively because split-finger transducers are more difficult to manufacture because of the fact that finger spacings are reduced by a factor of 2 for filters of a predetermined cut-off frequency or one has to be satisfied with a cut-off frequency of only half this value.
The low-loss filter according to U.S. Pat. No. 4,468,642 is such a filter in a two-track arrangement. For this filter, material having a high coupling factor is used, in particular, but this leads to interfering effects resulting from internal reflections in the transducers being particularly strong. Such a known low-loss filter consists of at least six transducer structures and, as a rule, has four further structures by means of which a unidirectional effect is effected for the four end transducers at the ends of the two tracks.
If the coupling transducers are dimensioned, as is the subject matter of U.S. Pat. No. 4,468,642, in the manner specified there with regard to the number of their interdigital single fingers, a filter having a minimum transmission or insertion loss is obtained. As specified there, however, such a low-loss filter has asymmetry and ripples in the transfer characteristic. The number 2N of transducer fingers specified in accordance with the U.S. Patent also defines the bandwidth according to the substrate material.
It has been found that if the number N of the (single) finger pairs of the coupling structures deviates from the stipulation of the U.S. Patent, a considerable increase in the ripple occurs so that, in practice, such measures are not taken.
Using the invention, a filter of the type specified in principle in U.S. Pat. No. 4,468,642 can be considerably improved, namely with regard to symmetry of the transfer characteristic between input and output of this filter. This also applies to coupling transducers having a number N of transducer finger pairs deviating from the stipulation of the U.S. Patent.